1. Field of the Invention
This invention relates to an office machine, and more particularly to a duplex document feeder cooperating with a scanning unit so as to perform a two-sided scanning operation.
2. Description of the Related Art
Referring to FIG. 1, an auto-document feeder disclosed in U.S. Pat. No. 5,784,680 is employed for an image forming apparatus 1, and includes a housing unit 2, a first transfer path 301 disposed in the housing unit 2, a second transfer path 302 in spatial communication with the first transfer path 301, an inverting path 303 disposed above the second transfer path 302, a switchback path 304 intercommunicating and external to the second transfer path 302 and the inverting path 303, a document-feeding roller unit 4 aligned with the first transfer path 301, a first conveying roller unit 501 disposed in the first transfer path 301, a second conveying roller unit 502 disposed in the second transfer path 302, an ejecting roller unit 6 disposed between the second transfer path 302 and the inverting path 303, a switchback roller unit 7 external to the ejecting roller unit 6, a first switching gate 8 disposed between the ejecting roller unit 6 and the switchback roller unit 7, and an inverting unit 9 internal to the ejecting roller unit 6. The housing unit 2 includes a document-setting tray 201, a guiding tray 202 disposed under the document-setting tray 201, and an ejection tray 203. The first switching gate 8 is pivotable between an upper position shown by the solid lines and proximate to the switchback path 304, and a lower position shown by the phantom lines and distal from the switchback path 304. The inverting unit 9 includes a curved path 901′ adjacent to the ejecting roller unit 6, and a second switching gate 902′ disposed at a junction between the curved path 901′ and the inverting path 303. The second switching gate 902′ is pivotable into the inverting path 303 so as to interrupt communication between the inverting path 303 and the switchback path 304, and is removable from the inverting path 303 so as to allow for communication between the inverting path 303 and the switchback path 304.
When it is desired to perform a two-sided scanning operation, a stack of documents is placed on the document-setting tray 201 in such a manner that first surfaces (I) of the documents face upwardly. The first switching gate 8 is initially disposed in the lower position. When a first sensor unit 901 detects the presence of the documents on the document-setting tray 201 to thereby output a signal to a controller (not shown), the document-feeding roller unit 4 is driven by the controller to move the uppermost document into the first transfer path 301. Also under control of the controller, the first conveying roller unit 501 moves the uppermost document to pass through a second sensor unit 902 and a scanning window 101 of the image forming apparatus 1. Upon receiving a signal from the second sensor unit 902, the controller activates a scanning unit 102 so as to perform a scanning operation on the first surface (I) of the uppermost document. The uppermost document is moved from the second transfer path 302 into the switchback path 304 through operation of the second conveying roller unit 502 and the ejecting roller unit 6, and subsequently toward the guiding tray 202 through operation of the switchback roller unit 7. At this time, the first surface (I) of the uppermost document faces downwardly, and a second surface (II) opposite to the first surface (I) faces upwardly.
Thereafter, when a third sensor 903 detects alignment of a trailing end of the uppermost document therewith, or when a fourth sensor 904 detects that the trailing end of the uppermost document has passed therethrough for a predetermined time period, under control of the controller, the rotational direction of each of rollers of the switchback roller unit 7 is reversed, and the first switching gate 8 is pivoted to the upper position so as to move the uppermost document into the first transfer path 301 via the inverting path 303.
When the uppermost document passes through the second sensor 902 once again, a scanning operation on the second surface (II) is performed, and the first switching gate 8 is pivoted to the lower position. Through operation of the second conveying roller unit 502 and the ejecting roller unit 6, the uppermost document is moved again into the switchback path 304 such that the first surface (I) faces upwardly, and the second surface (II) faces downwardly.
Thereafter, when the third sensor unit 903 detects alignment of the trailing end of the uppermost document therewith, under control of the controller, the rotational direction of each of the rollers of the switchback roller unit 7 is reversed, and the second switching gate 902′ is pivoted to the upper position (i.e., extends into the inverting path 303). Hence, the uppermost document moves along the curved path 901′, and drops onto the ejection tray 203. As a result, the first surface (I) of the uppermost document faces downwardly, and the second surface (II) faces upwardly. Thus, when all of the documents are ejected, they are arranged orderly.
The auto-document feeder has the following disadvantages:    (1) To invert the documents after completion of two-sided scanning, a large number of elements are needed to cooperate with the curved path 901′ and the second switching gate 902′, thereby resulting in a complicated structure.    (2) The curvature radius of the curved path 901′ is small. Therefore, it is difficult for the documents to be ejected fully. In other words, paper jams easily occur. Furthermore, when the documents pass through the ejecting roller unit 6 to thereby drop onto the ejection tray 203, they are easily curled.    (3) During operation, the documents are moved through an annular document path, which includes the switchback path 304, the inverting path 303, the second transfer path 302, and the switchback path 304, and which allows enter and exit of the documents via a nip defined by two rollers of the switchback roller unit 7. Provided that the length of the annular document path is shorter than the documents, when a two-sided scanning operation is performed on one document, the leading and trailing ends thereof may overlap each other on the switchback roller unit 7, thereby resulting in a paper jam. Thus, the length of the annular document path (P) must be longer than the largest document that the image forming apparatus is designed to handle. This increases significantly the whole length and volume of the duplex document feeder.